GRE阅读练习每日一篇(四十七)
<p> 编辑点评： <span word="GRE">GRE</span>阅读的方法相信大家已经积累了很多了，本文就为大家提供一些<span word="GRE">GRE</span>阅读的材料，大家来练一练，把平时学到的东西运用到实际中，也多多积累词汇和句子，提高自己的阅读能力。</p><p> 每天做一些标准的<span word="GRE">GRE</span>阅读练习，有助于大家在<span word="GRE">GRE</span>考试的复习过程中不断地进行练习和总结。希望大家在进行<span word="GRE">GRE</span>阅读练习时，充分运用平时所积累的知识，这样才能有效果。</p><p> <span word="Classical">Classical</span> <span word="physics">physics</span> <span word="defines">defines</span> <span word="the">the</span> <span word="vacuum">vacuum</span> <span word="as">as</span> <span word="a">a</span> <span word="state">state</span> <span word="of">of</span> <span word="absence">absence</span>: <span word="a">a</span> <span word="vacuum">vacuum</span> <span word="is">is</span> <span word="said">said</span> <span word="to">to</span> <span word="exist">exist</span> <span word="in">in</span> <span word="a">a</span> <span word="region">region</span> <span word="of">of</span> <span word="space">space</span> <span word="if">if</span> <span word="there">there</span> <span word="is">is</span> <span word="nothing">nothing</span> <span word="in">in</span> <span word="it">it</span>. <span word="In">In</span> <span word="the">the</span> <span word="quantum">quantum</span> <span word="field">field</span> <span word="theories">theories</span> <span word="that">that</span> <span word="describe">describe</span> <span word="the">the</span> <span word="physics">physics</span> <span word="of">of</span> <span word="elementary">elementary</span> <span word="particles">particles</span>, <span word="the">the</span> <span word="vacuum">vacuum</span> <span word="becomes">becomes</span> <span word="somewhat">somewhat</span> <span word="more">more</span> <span word="complicated">complicated</span>. <span word="Even">Even</span> <span word="in">in</span> <span word="empty">empty</span> <span word="space">space</span>, <span word="particles">particles</span> <span word="can">can</span> <span word="appear">appear</span> <span word="spontaneously">spontaneously</span> <span word="as">as</span> <span word="a">a</span> <span word="result">result</span> <span word="of">of</span> <span word="fluctuations">fluctuations</span> <span word="of">of</span> <span word="the">the</span> <span word="vacuum">vacuum</span>. <span word="For">For</span> <span word="example">example</span>, <span word="an">an</span> <span word="electron">electron</span> <span word="and">and</span> <span word="a">a</span> <span word="positron">positron</span>, <span word="or">or</span> <span word="antielectron">antielectron</span>, <span word="can">can</span> <span word="be">be</span> <span word="created">created</span> <span word="out">out</span> <span word="of">of</span> <span word="the">the</span> <span word="void">void</span>. <span word="Particles">Particles</span> <span word="created">created</span> <span word="in">in</span> <span word="this">this</span> <span word="way">way</span> <span word="have">have</span> <span word="only">only</span> <span word="a">a</span> <span word="fleeting">fleeting</span> <span word="existence">existence</span>; <span word="they">they</span> <span word="are">are</span> <span word="annihilated">annihilated</span> <span word="almost">almost</span> <span word="as">as</span> <span word="soon">soon</span> <span word="as">as</span> <span word="they">they</span> <span word="appear">appear</span>, <span word="and">and</span> <span word="their">their</span> <span word="presence">presence</span> <span word="can">can</span> <span word="never">never</span> <span word="be">be</span> <span word="detected">detected</span> <span word="directly">directly</span>. <span word="They">They</span> <span word="are">are</span> <span word="called">called</span> <span word="virtual">virtual</span> <span word="particles">particles</span> <span word="in">in</span> <span word="order">order</span> <span word="to">to</span> <span word="distinguish">distinguish</span> <span word="them">them</span> <span word="from">from</span> <span word="real">real</span> <span word="particles">particles</span>, <span word="whose">whose</span> <span word="lifetimes">lifetimes</span> <span word="are">are</span> <span word="not">not</span> <span word="constrained">constrained</span> <span word="in">in</span> <span word="the">the</span> <span word="same">same</span> <span word="way">way</span>, <span word="and">and</span> <span word="which">which</span> <span word="can">can</span> <span word="be">be</span> <span word="detected">detected</span>. <span word="Thus">Thus</span> <span word="it">it</span> <span word="is">is</span> <span word="still">still</span> <span word="possible">possible</span> <span word="to">to</span> <span word="define">define</span> <span word="that">that</span> <span word="vacuum">vacuum</span> <span word="as">as</span> <span word="a">a</span> <span word="space">space</span> <span word="that">that</span> <span word="has">has</span> <span word="no">no</span> <span word="real">real</span> <span word="particles">particles</span> <span word="in">in</span> <span word="it">it</span>.</p><p> <span word="One">One</span> <span word="might">might</span> <span word="expect">expect</span> <span word="that">that</span> <span word="the">the</span> <span word="vacuum">vacuum</span> <span word="would">would</span> <span word="always">always</span> <span word="be">be</span> <span word="the">the</span> <span word="state">state</span> <span word="of">of</span> <span word="lowest">lowest</span> <span word="possible">possible</span> <span word="energy">energy</span> <span word="for">for</span> <span word="a">a</span> <span word="given">given</span> <span word="region">region</span> <span word="of">of</span> <span word="space">space</span>. <span word="If">If</span> <span word="an">an</span> <span word="area">area</span> <span word="is">is</span> <span word="initially">initially</span> <span word="empty">empty</span> <span word="and">and</span> <span word="a">a</span> <span word="real">real</span> <span word="particle">particle</span> <span word="is">is</span> <span word="put">put</span> <span word="into">into</span> <span word="it">it</span>, <span word="the">the</span> <span word="total">total</span> <span word="energy">energy</span>, <span word="it">it</span> <span word="seems">seems</span>, <span word="should">should</span> <span word="be">be</span> <span word="raised">raised</span> <span word="by">by</span> <span word="at">at</span> <span word="least">least</span> <span word="the">the</span> <span word="energy">energy</span> <span word="equivalent">equivalent</span> <span word="of">of</span> <span word="the">the</span> <span word="mass">mass</span> <span word="of">of</span> <span word="the">the</span> <span word="added">added</span> <span word="particle">particle</span>. <span word="A">A</span> <span word="surprising">surprising</span> <span word="result">result</span> <span word="of">of</span> <span word="some">some</span> <span word="recent">recent</span> <span word="theoretical">theoretical</span> <span word="investigations">investigations</span> <span word="is">is</span> <span word="that">that</span> <span word="this">this</span> <span word="assumption">assumption</span> <span word="is">is</span> <span word="not">not</span> <span word="invariably">invariably</span> <span word="true">true</span>. <span word="There">There</span> <span word="are">are</span> <span word="conditions">conditions</span> <span word="under">under</span> <span word="which">which</span> <span word="the">the</span> <span word="introduction">introduction</span> <span word="of">of</span> <span word="a">a</span> <span word="real">real</span> <span word="particle">particle</span> <span word="of">of</span> <span word="finite">finite</span> <span word="mass">mass</span> <span word="into">into</span> <span word="an">an</span> <span word="empty">empty</span> <span word="region">region</span> <span word="of">of</span> <span word="space">space</span> <span word="can">can</span> <span word="reduce">reduce</span> <span word="the">the</span> <span word="total">total</span> <span word="energy">energy</span>. <span word="If">If</span> <span word="the">the</span> <span word="reduction">reduction</span> <span word="in">in</span> <span word="energy">energy</span> <span word="is">is</span> <span word="great">great</span> <span word="enough">enough</span>, <span word="an">an</span> <span word="electron">electron</span> <span word="and">and</span> <span word="a">a</span> <span word="positron">positron</span> <span word="will">will</span> <span word="be">be</span> <span word="spontaneously">spontaneously</span> <span word="created">created</span>. <span word="Under">Under</span> <span word="these">these</span> <span word="conditions">conditions</span> <span word="the">the</span> <span word="electron">electron</span> <span word="and">and</span> <span word="positron">positron</span> <span word="are">are</span> <span word="not">not</span> <span word="a">a</span> <span word="result">result</span> <span word="of">of</span> <span word="vacuum">vacuum</span> <span word="fluctuations">fluctuations</span> <span word="but">but</span> <span word="are">are</span> <span word="real">real</span> <span word="particles">particles</span>, <span word="which">which</span> <span word="exist">exist</span> <span word="indefinitely">indefinitely</span> <span word="and">and</span> <span word="can">can</span> <span word="be">be</span> <span word="detected">detected</span>. <span word="In">In</span> <span word="other">other</span> <span word="words">words</span>, <span word="under">under</span> <span word="these">these</span> <span word="conditions">conditions</span> <span word="the">the</span> <span word="vacuum">vacuum</span> <span word="is">is</span> <span word="an">an</span> <span word="unstable">unstable</span> <span word="state">state</span> <span word="and">and</span> <span word="can">can</span> <span word="decay">decay</span> <span word="into">into</span> <span word="a">a</span> <span word="state">state</span> <span word="of">of</span> <span word="lower">lower</span> <span word="energy">energy</span>; <span word="i">i</span>.<span word="e">e</span>., <span word="one">one</span> <span word="in">in</span> <span word="which">which</span> <span word="real">real</span> <span word="particles">particles</span> <span word="are">are</span> <span word="created">created</span>.</p>
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